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Ligands such as complexing organic acids play a significant role in the dynamics of release of plant nutrients in soils. The objective of this work was to study the dynamics of slowly exchangeable K and Mg release from 12 Quebec soils by ten successive extractions with 5 × 10−4 M citric acid, over a period of 1800 h, and by a single extraction with boiling 1.0 M HNO3. All soils were characterized by a mixed clay mineralogical composition with swelling minerals as major components. The kinetics of slowly exchangeable K and Mg cumulative release were described using two forms of the Elovich, the parabolic diffusion, power function, and first and zero order equations. Comparisons of r and SE values showed that the extended form of the Elovich equation best fitted K and Mg release to citric acid. The amount and rate of K release by the citric acid procedure were highly correlated to the amounts of slowly exchangeable HNO3-extractable and rapidly exchangeable NH4OAc-extractable K and to the coarse clay content of the soils. The amount of Mg released to citric acid was proportional to the unbuffered cation exchange capacity of the soils. The results indicated that the citric acid procedure was a time saving alternative to the study of the kinetics of slowly exchangeable K and Mg release from soils. Key words: Nonexchangeable K, citric acid, nonexchangeable Mg, podzolic soils, kinetics

Regression analyses were used to correlate organic matter, texture and surface area values to the cation‐exchange capacity (CEC) of clay‐rich soils in the lowlands of Quebec. The Ap, Bg and Cg horizons of 11 Gleysolic soils (Aqu‐Suborders) were analyzed for CEC, exchangeable bases and acidity, total and fine clay contents, organic matter, surface area and mineralogical composition. The soils contained illite, chlorite, smectite, and vermiculite in addition to quartz and feldspars. In the Ap horizon, total clay (r = 0.815**) and fine clay (r = 0.841**) contents were better related to CEC than organic matter (r = 0.566**) and surface area (r = 0.570**). In the Bg and Cg horizons, surface area (r = 0.918** and 0.814**) gave the best correlation with CEC. As calculated by multiple regression equations, CEC values of organic matter increased with depth from 56.5 to 223 meq/100 g, while CEC values of total clay decreased from 37.7 to 22.5 meq/100 g and that of fine clay decreased from 57.5 to 50.7 meq/100 g. Variations in the mineralogical composition, although small, were sufficient to explain nearly 50% of the variations in the CEC. Clay mineralogy is important to predict the CEC of such soils where the contribution of clay is 3.5 to 5 times greater than that of organic matter.

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